Light sources having high brightness, low consumption power, and long life are desired in projection display apparatuses represented by projectors. Some exemplary light sources satisfying such requests are light emitting diodes (LED) and laser diodes (LD).
FIG. 13 is a block diagram of a projection display apparatus 100A which utilizes LED as a light source. Light rays from a green LED 101G, a blue LED 101B, and a red LED 101R are respectively transmitted through lenses 107a-107f, light modulators 102G, 102B, and 102R. Thereafter, respective light rays are combined by a cross dichroic prism 103, incident on a projection lens 108, and then projected onto a screen or the like (not illustrated in the drawing) from the projection lens 108.
Note that light modulators 102G, 102B, and 102R include polarizers, liquid crystal cells and analyzers, and spatially modulate and emit light incident thereon.
The cross dichroic prism 103 includes a dichroic film reflecting only light in a blue wavelength band, and a dichroic film reflecting only light in a red wavelength band. Then, the green light, the blue light, and the red light incident on the cross dichroic prism 103 from the three directions are combined, and projected onto the projection lens 108.
It is known that such an LED is made of a semiconductor, and can emit blue light by using an InGaN-based semiconductor material and red light by using an AlGaInP-based semiconductor material. However, an LED emitting green light formed by using InGaN-based and AlGaInP-based semiconductor materials suffers a problem of having low light emitting efficiency. This problem is called “green gap.”
A light source combining LED or LD light sources with a phosphor light source has been proposed as a method to solve the lack in light quantity of green light related to this green gap. As an example of a light source using a phosphor, a configuration in which a blue LED and a phosphor are combined has been known (so-called “phosphor LED”). In this configuration, the blue light emitted from the blue LED is used as excitation light which helps the phosphor generate green light.
The blue LED used as an excitation light source has a larger output than the green LED does. Therefore, by using a phosphor LED, it is possible to obtain green light of higher brightness than that of green light emitted from a green LED.
On the other hand, Patent Literature 1 discloses a method to emit green light of high brightness using a plurality of green LEDs. FIG. 14 shows a configuration of the projection display apparatus 100B described in Patent Literature 1.
This projection display apparatus 100B includes a green LED 101G, a combined LED 110B, and a red LED 101R. Note that the combined LED 110B includes a blue LED 101B and a green LED 111G. The combined LED 110B emits light in which blue light and green light are combined.
Light rays respectively from LEDs are transmitted through lenses 107a-107f, light modulators 102G, 102B, and 102R, and incident on a cross dichroic prism 103.
The cross dichroic prism 103 has wavelength dependence and polarization dependence, and combines green light, blue light, and red light, as well as uses polarization to combine green light emitted from the green LED 101G and green light emitted from the green LED 111G of the combined LED 110B.
By emitting blue light and green light from the combined LED 110B, the intensity of green light emitted from the projection lens 108 can be increased.
Note that as a method to further increase the intensity of green light, it is possible to use a phosphor LED in the projection display apparatus of Patent Literature 1.